The invention relates to the cooling of modular electronic devices, especially but not exclusively to xe2x80x9con boardxe2x80x9d devices placed on board aircraft, or tanks, etc.
xe2x80x9cOn boardxe2x80x9d devices must operate under harsh environmental conditions (vibrations, shocks, humidity, accelerations, heat dissipation difficulty).
These difficult conditions may cause breakdowns. This means that these electronic units have to be designed for the purpose of making their repair and their maintenance easier.
One of the new aeronautical standards stipulates that they be produced in the form of a modular structure called LRM (Line Replaceable Module), such structures being installed in racks.
A rack may include a large number of electronic modules. This makes rapid maintenance, handling and repair work easy, but tends to make it more difficult to extract the heat produced by the various components contained in the modules. These components, of increasing sophistication, produce increasing amounts of heat. The inventions aims to solve the problem of extracting this heat.
FIG. 1 shows in perspective a rack 1 containing several electronic modules. The rack 1 is generally parallelepipedal, with a front face 3 for inserting and extracting the electronic modules 2a to 2e and five other faces 4, 5, 6, 6a, 7. The face 7 consists of a back plate which closes off the rack 1 and carries connectors into which the corresponding connectors of the modules 2a to 2e are plugged.
The bottom and top plates 4, 5 have slideways, for example made of metal, respectively 9, 9a, for guiding the electronic modules 2a to 2e in the rack 1 and for keeping them therein.
The rack 1 may include openings for air circulation, for the purpose of cooling the electronic modules 2a to 2e. The cooling air (shown symbolically by arrows labeled 30) is generally injected into the rack via the bottom plate 4, through openings 32 called air inlet openings located between the slideways 9. The cooling air 30 circulates between the modules 2a to 2e (and possibly in these modules). The heat-laden air 33 leaves the rack 1 through outlet openings 34, formed between the slideways 9a and the top plate 5.
FIG. 2 shows in a simplified manner, in cross section, an example of a conventional electronic module structure, as may be found, for example, in patent GB-A-2 270 207.
The electronic module 2e comprises two covers, 18, 19, for example made of aluminum, having a thickness of the order of 1 millimeter. Aluminum is a particularly advantageous material from the standpoint of on board mass, in that it has both a low density and a very good mechanical rigidity even with a small thickness; it also has a moderate thermal conductivity, although very much less than that of copper.
A printed circuit board 15 is placed in the space bounded by the two covers 18, 19. It is gripped around its periphery by the edges of the covers. The printed circuit board 15 bears the various components of an electronic circuit, among which at least one component 22 is producing a large amount of heat in operation (for example a powerful microprocessor).
In the example in FIG. 2, the component 22 has a thickness e1 which corresponds approximately to a distance d1 between the inside of the first cover 18 and that face (face 23) of the printed circuit which bears the component. The component 22 is thus in practice directly in contact with the first cover 18 via, for example, a thermal interface or matching layer 25 which promotes thermal contact and is electrically insulating (an elastomer or epoxy resin).
Thus, the covers 18, 19, and particularly the first cover 18 in the example shown, provide, in addition to their function of protecting the printed circuit 15 and of electrostatic shielding, a heat sink function.
The heat transmitted to the cover is itself extracted by conduction to the slideways 9, 9a of the rack and then to the walls of the rack; the heat is also extracted by convection, thanks to the cooling air 30 coming from the abovementioned ventilation. Ventilation inside the electronic modules 2a to 2e may be added, by providing bottom and top openings 26, 27 in the bottom and top walls 18a, 18b and 19a, 19b of the covers, respectively.
Even when the three means mentioned above are combined, the extraction of heat may be insufficient with current components. One of the aims of the present invention is to improve this extraction.
For this purpose, the invention provides an electronic module of the type intended to operate in a rack, comprising at least one printed circuit board, at least one protective cover defining a housing for the board, at least one component producing heat in operation and mounted via its bottom face on the board, and a thermal link between a top face of the component and the cover, characterized in that the thermal link comprises, interposed between the top face of the component and the cover, a device having a high thermal conductivity, this device having an area greater than that of the component and having a thermal conductivity greater than that of the cover.
The word xe2x80x9cinterposedxe2x80x9d should be understood to mean that the device is at least partly placed in series between the component and the cover in the thermal circuit of heat flow from the component to the cover. This even applies if the device having a high thermal conductivity also includes a part which extracts the heat directly to the natural air or to the forced circulation air which lies outside the cover. However, in a preferred embodiment, the device having a high thermal conductivity is physically interposed in its entirety between the component and the cover and is applied over its entire area (which is greater than that of the component) against this cover.
The term xe2x80x9cdevice having a high thermal conductivityxe2x80x9d, or in short xe2x80x9cHTC devicexe2x80x9d, should be understood to mean any device or element having a better thermal conductivity than that of the material of which a cover responsible for extracting the heat is made, so as to make it possible to even out or tend to even out the temperature at all points on a surface of the cover with which this HTC device may be brought into contact. They may, for example, when the cover is made of aluminum, be elements based on one or more materials whose thermal conductivity is greater than or equal to that of copper, or else devices which involve changes of phase of a solid, liquid or gaseous element allowing substantial amounts of energy to be transported; some of the latter devices are known especially as xe2x80x9cheat pipesxe2x80x9d and may consist of a hollow plate containing a liquid, the cooling relying on the energy consumed by the liquid-to-gas phase change in a closed circuit in the hollow plate. In this case, it will be understood that one speaks of an equivalent thermal conductivity, corresponding to the heat extraction capability: the equivalent conductivity is that of an imaginary material which, having the dimensions of the device (for example the heat pipe), would have the same heat extraction capability.
The application of an HTC device to a region of a cover makes this region better able to conduct heat and, as it were, better able to distribute it to the rest of the cover which, although a poorer thermal conductor, then benefits from a larger area conducive to this conduction. This results overall in an increase in the thermal conductivity of the cover, in a ratio very much greater than that of the increase in the mass of the cover resulting from the presence of the HTC device.